7. Arya Lets The Hound Die

This one is cheating a bit, because Arya doesn't actually apply the coup de grace, the Hound having been ostensibly mortally wounded in the battle with Brienne. But the lack of kill makes this even more harsh. Arya and the Hound became the show's favorite odd couple in Season 4, seemingly bonding at times with the Hound taught her how to stay alive. As the Hound begs to be killed, Arya refuses, letting him know two things: 1. Their friendship was really just based on her survival and 2. She learned his lessons about being hard to survive all too well.